Method for recovering metal ions in aqueous liquid

ABSTRACT

Esters of comenic acid were found to form metal chelate compounds with various metal ions and these chelate compounds are soluble in organic solvents. This finding makes it possible to recover or remove metal ions from industrial drainage and the like.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of Ser. No.350,846, KAZUNOBU NAKATSUKA ET AL, filed Apr. 13, 1973, for "METHOD FORRECOVERING METAL IONS IN AQUEOUS LIQUID", and now abandoned.

This invention relates to a method for recovering and/or removing metalions from a water solution. Recently, pollution particularly sea orriver water pollution by heavy metal ions has constituted one of mostimportant social problems since said pollution may jeoparadize thesafety in daily life of citizen. Therefore, a number of counterplans toprevent the pollution have been considered in various parts of country.

As the result of a study to eliminate metal ions in water, the presentinventors have found that some comenic esters easily soluble in organicsolvents can be effective to this purpose and accomplished the presentinvention. In the present invention metal cations can be recovered fromits aqueous solution as a form of corresponding metal chelate compoundsof comenic esters by contacting the aqueous solution at a pH of not lessthan 3 with comenic esters wherein the alcohol group contains at least 3carbon atoms dissolved in a hydrophobic organic solvent to transfer theformed chelate compounds to the organic layer.

As for the sorts of comenic esters which can be used in the presentinvention, the following examples are given; a lower alkyl ester (suchas propyl, butyl, isobutyl, amyl, or isoamyl), middle to higher alkylester (such as octyl, nonyl, lauryl, or myristyl), an aralkyl ester suchas benzyl, and a cycloalkyl ester such as cyclohexyl. Out of them somehigher alkyl esters are more preferable for the present purpose.

Any of these esters can easily form a chelate compound together withmetal ions (such as Fe, Cu, Cd, Pb, Ni, Co, Zn, Sn, Au, Pt, Al, Sb, Bi,Cr, Y and oxides of uranium group for example UO₂) in water and eachformed chelate compound is easily soluble in organic solvents. Forexample, the solubility at 25° C of various metal chelate compoundsformed with octyl comenate are shown in Table 1.

                                      Table 1                                     __________________________________________________________________________    sol-                                                                          vent                                                                          che-              1,2-di-                                                                            methyl-                                                                            ethyl                                             late  ben-                                                                              tolu-                                                                             xy- chloro-                                                                            ethyl-                                                                             ace-                                                                              chloro-                                       compound                                                                            zene                                                                              ene lene                                                                              ethane                                                                             ketone                                                                             tate                                                                              form                                          __________________________________________________________________________    Fe.sup.2+                                                                           +++ +++ +++ +++  +++  +++ +++                                           Sn.sup.2+                                                                           -   -   -   -    +    -   -                                             Co.sup.2+                                                                           ++  ++  ++  ++   +    +   ++                                            Ni.sup.2+                                                                           ++  ++  ++  ++   +    ++  ++                                            Cu.sup.2+                                                                           +   +   +   +    +    +   ++                                            Zn.sup.2+                                                                           ++  ++  ++  ++   +    +   ++                                            Cd.sup.2+                                                                           ++  ++  ++  +    +    +   ++                                            Al.sup.3+                                                                           ++  ++  ++  ++   ++   ++  ++                                            Pb.sup.2+                                                                           -   -   -   +    +    +   +                                             Y.sup.3+                                                                            +   +   +   +++  +    +   +++                                           UO.sub.2.sup.2+                                                                     ++  ++  ++  +++  +    +   +++                                           Sb.sup.2+                                                                           +   +   +   ++   +    +   ++                                            Bi.sup.2+                                                                           +   +   +   ++   +    +   ++                                            Au.sup.3+                                                                           -   -   -   ++   -    -   ++                                            Pt.sup.2+                                                                           +   +   +   +    -    -   +                                             Cr.sup.2+                                                                           +   +   +   +++  ++   ++  +++                                           __________________________________________________________________________     +++ : Very soluble (1 g/l ml or less)                                          ++ : Soluble (1 g/1-10 ml)                                                   +  : Slightly soluble (1 g/10-30 ml)                                          -  : Very slightly soluble (1 g/100-1000 ml)                             

Based on these findings the present inventive process is established asfollows: firstly, dissolving one of comenic esters mentioned above in ahydrophobic organic solvent, secondly, contacting the organic solutionwith an aqueous solution containing metal ions by shaking or stirringthe two layers to form the corresponding chelate compounds underarranging pH of the aqueous solution with alkali in order to controlexcessively acidic pH owing to the acid which is formed by a protonreleased from the comenic ester and a residual anion in the aqueoussolution, and then efficiently recovering the metal chelate compoundstransferred into the hydrophobic organic layer. Thus, the present methodmay be utilizable for recovery of noble metals or transuranium elementsin a solution as well as for recovery of heavy metals being noxious forthe human from the industrial drainage. Moreover it may be applicable toanalytical chemistry. Because the formed metal chelate compound producea specific color (shown in Table 2) which may be colorimetricallymeasured to determine the amount of the metal.

To remove or recover metal ions from water solution, experiments weremade according to the same procedure as in Examples 1 to 5 and variousmetal chelate compounds were obtained from the organic solvent. Theproperties of these chelate compounds are shown in Table 3. (With mostof the chelates of Table 3, the metals were recovered from the waterlayer with almost the same recovery ratio as in Examples 1 to 5.) Thechelate compounds may have the structure (I) or (II) shown below:##STR1## wherein M represents a divalent metal ion, M' represents atrivalent metal ion and R represents an alkyl or aralkyl group of atleast 3 carbon atoms.

Recovery of metal ions themselves from the organic solvent can beachieved by treating the isolated chelate compounds from the organiclayer with a mineral acid for example hydrochloric acid under cooling togive comenic esters and the metal chloride. As for the organic solvent,use can be made of any solvent which is not soluble in water, forexample, benzene, toluene, xylene, chloroform, 1,2-dichloroethane ortrichloroethylene. The preferable amount of comenic esters is two tothree moles or more per one mole of the metal ions.

In performance of the present process, it is necessary to suitablycontrol the pH of the water solution by addition of alkali so that thewater solution may not undergo strong acidification, since the watersolution becomes acidic with decrease of metal ions in the water layer.Allowable limit of the pH range varies depending upon the sort of theanions coexisting with the metal ions in the water. For example, in caseof a heavy metal salt of a strong acid (e.g. CuSO₄) it goes well at acomparatively lower pH, that is, of not less than 3, while in case of aweak anion salt (e.g. CH₃ COO⁻) the limit shifts to nearly neutral pH,that is of not less than 4. If the pH goes to acidic beyond these limit,the efficiency of recovery decreases due to opposite proceeding ofreaction equilibrium. Generally a range of not less than 3, particularly4 to 7 is preferable. Alkaline pH may cause precipitation of hydroxideswhen concentration of the heavy metal ions is considerably high.Therefore, it may be successful even in alkaline pH when concentrationsof the cation is not so high.

The performance of the present process is easily operable and possibleto remove several ions present in a solution at the same time. Exampleof efficiency in recovery of metal ions according to the presentinvention are listed in the following Table 2. If it is necessary ordesired to remove cations to a concentration lower than those shown inTable 2, a repetition of the same process or a combination with otherprocesses would be recommendable for the economical performance.

                                      Table 2                                     __________________________________________________________________________    Metal ions Comenic esters  Concent-                                                                      ration in                                               con-             Mole water                                                   cent-      concent-                                                                            ratio                                                                              after   Recovery                                        ration                                                                              sort of                                                                            ration                                                                              metal:                                                                             treatment                                                                             ratio                                      sort (%)   alcohol                                                                            in CHCl.sub.3                                                                       ester                                                                              (%)     (%)                                        __________________________________________________________________________    Fe.sup.2+                                                                          0.224 amyl 1.35  1:3  3.104 × 10.sup.-3                                                               98.61                                      Fe.sup.2+                                                                          0.112 amyl 1.35  1:6  1.111 × 10.sup.-3                                                               98.61                                      Fe.sup.2+                                                                          0.056 amyl 1.35   1:12                                                                              0.824 × 10.sup.-3                                                               98.53                                      Cu.sup.2+                                                                          0.198 octyl                                                                              1.67  1:2  0.915 × 10.sup.-3                                                               99.79                                      Pb.sup.2+                                                                          0.311 amyl 1.35  1:2  0.5 × 10.sup.-3                                                                 99.84                                      Cd.sup.2+                                                                          0.348 octyl                                                                              1.67  1:2  7.5 × 10.sup.-3                                                                 97.85                                      Cd.sup.2+                                                                          0.02  lauryl                                                                             1.80  1:2  0.488 × 10.sup.-3                                                               97.56                                      Cu.sup.2+                                                                          0.02  lauryl                                                                             1.80  1:2  0.27 × 10.sup.-3                                                                98.65                                      Pb.sup.2+                                                                          0.02  lauryl                                                                             1.80  1:2  0.90 × 10.sup.-3                                                                95.50                                      __________________________________________________________________________

Table 2 shows that metal ions can be efficiently extracted with comenicesters in organic solvents. Quantitative analysis of Fe²⁺ was doneaccording to the colorimetric method using a nitroso R salt (Keiji Yano,The Kagaku no Ryoiki, Extra number 33, Electrophoto-colorimetry (1) page23 Nankodo: Saneo Noda, Biochemistry volume 29, page 199). Other metalions were quantitatively analyzed according to the atomic absorptionanalysis method using Atomic Absorption Analizer Type 208 made byHitachi Seisakusho. (Correspondence of wave lengths and metal ions areas follows: Cu⁺² ; 3247 A, Cd⁺² : 2288 A, and Pb⁺² : 2833 A).

The recovery ratios are calculated according to the following equation:

The following examples will explain the present invention in moredetail.

EXAMPLE 1

In 200 ml of chloroform was dissolved 3.34 g of octyl comenate. 1.5475 gof copper sulfate (CuSO₄ ·H₂ O) was dissolved in 200 ml of desaltedwater (0.0395 g of cuppric ion is contained in 20 ml of this solution).Each 20 ml of both solutions was combined and shaken enough. The pH ofthe aqueous layer was kept at 4.0 with 1% aqueous ammonium solutionduring the shaking. When the pH became constant without any addition ofammonium, the water layer was separated from the chloroform layer andmade to 40 ml of volume with water, in which total cuppric ion contentwas determined to be 0.0183 × 10⁻² g. (Recovery ratio: 99.79%).

EXAMPLE 2

In 200 ml of desalted water was dissolved 1.3 g of cadmium sulfate(CdSO₄). 20 ml of this solution (containing 0.06975 g of Cd⁺² ion) wascontained and shaken enough with 20 ml of octyl comenate toluenesolution whose concentration was same as in Example 1. The pH of theaqueous layer was kept at 4.6. In the same manner as in Example 1 theamount of the Cd⁺² ion was examined to be 0.0015 g.

EXAMPLE 3

In 200 ml of desalted water was dissolved 1.138 g of lead acetate(Pb(CH₃ COO)₂ 3H₂ O). 2.715 g of amyl comenate was dissolved in 200 mlof 1,2-dichloroethane. (0.0622 g of lead ion is contained in 20 ml ofthis solution.) Each 20 ml of the both solutions was combined and shakenenough. In the same manner as in Example 1 the amount of lead ion wasdetermined to be 0.01 × 10⁻² g. (recovery ratio: 99.84%).

EXAMPLE 4

In desalted water was dissolved 1.11212 g of ferrous sulfate (FeSO₄ ·7H₂O) and made a volume of 100 ml. 2.715 g of amyl comenate was dissolvedin 200 ml of trichloroethylene. 20 ml of this solution was each combinedwith 5 ml (Fe⁺² ; 0.011195 g), 10 ml (Fe⁺² ; 0.02239 g) and 20 ml (Fe⁺²; 0.04478 g) of the ferrous solution above prepared. In the same manneras in Example 1 each amount of ferrous ions was determined as follows:

    ______________________________________                                        Volume   Amount of the Fe.sup.+2                                                                        Recovery ratio (%)                                  ______________________________________                                         5 ml    0.1648 × 10.sup.-3                                                                       98.53                                               10 ml    0.222  × 10.sup.-3                                                                       98.61                                               20 ml    0.6208 × 10.sup.-3                                                                       98.61                                               ______________________________________                                    

EXAMPLE 5

In desalted water were dissolved 39.29 mg of cuppric sulfate, 18.54 mgof cadmium sulfate and 18.31 mg of lead acetate and made a volume of 50ml (in which 10 mg of each metal ion was contained). In 50 ml ofchloroform was dissolved 900 mg of lauryl comenate. The both solutionswere combined and shaken enough. The pH of the water layer was kept at3.8 with aqueous ammonia during the shaking. When the pH became constantwithout any addition of ammonia, the water layer was separated from thechloroform layer and made a volume of 100 ml with water, in which thecontent of cuppric, cadmium and lead ions were 0.135 × 10⁻³ g (recoveryratio; 98.65%), 0.244 × 10⁻³ g (recovery ratio; 97.56%) and 0.45 × 10⁻³g (recovery ratio; 95.5%), respectively.

                                      Table 3                                     __________________________________________________________________________                Melting                                                           Metal chelate compound                                                                    point                                                             ester part                                                                          metal ion                                                                           (° C)                                                                         Rational formula                                                                        Color                                            __________________________________________________________________________    octyl Fe.sup.3+                                                                            108     (C.sub.14 H.sub.19 O.sub.5).sub.3                                                     dark red                                         octyl Sn.sup.2+                                                                            118   Sn(C.sub.14 H.sub.19 O.sub.5).sub.2                                                     light yellow                                     octyl Co.sup.2+                                                                           >245   Co(C.sub.14 H.sub.19 O.sub.5).sub.2                                                     reddish brown                                    octyl Ni.sup.2+                                                                           >223   Ni(C.sub.14 H.sub.19 O.sub.5).sub.2                                                     yellowish                                                                     brown                                            octyl Cu.sup.2+                                                                           >184   Cu(C.sub.14 H.sub.19 O.sub.5).sub.2                                                     greenish                                                                      yellow                                           octyl Zn.sup.2+                                                                           >241   Zn(C.sub.14 H.sub.19 O.sub.5).sub.2                                                     light yellow                                     octyl Al.sup.3+                                                                            112   Al(C.sub.14 H.sub.19 O.sub.5).sub.3                                                     light yellow                                     octyl Cd.sup.2+                                                                           >245   Cd(C.sub.14 H.sub.19 O.sub.5).sub.2                                                     light yellow                                     octyl UO.sub.2.sup.2+                                                                      179   UO.sub.2 (C.sub.14 H.sub.19 O.sub.5).sub.2                                              orange                                           octyl Y.sup.3+                                                                            >300   Y(C.sub. 14 H.sub.19 O.sub.5).sub.3                                                     light yellow                                     methyl                                                                              Al.sup.3+                                                                            142   Al(C.sub.7 H.sub.6 O.sub.5).sub.3                                                       colorless                                        methyl                                                                              Ni.sup.2+                                                                           >245   Ni(C.sub.7 H.sub.6 O.sub.5).sub.2.2H.sub.2 O                                            yellowish                                                                     brown                                            methyl                                                                              Zn.sup.2+                                                                           >254   Zn(C.sub.7 H.sub.6 O.sub.5).sub.2.2H.sub.2 O                                            yellowish                                                                     brown                                            butyl Cu.sup.2+                                                                           >192   Cu(C.sub.10 H.sub.11 O.sub.5).sub.2                                                     greenish                                                                      yellow                                           butyl Co.sup.2+                                                                           >300   Co(C.sub.10 H.sub.11 O.sub.5).sub.2                                                     reddish                                                                       brown                                            isobutyl                                                                            Cu.sup.2+                                                                           >184   Cu(C.sub.10 H.sub.11 O.sub.5).sub.2                                                     greenish                                                                      blue                                             isobutyl                                                                            Co.sup.2+                                                                           >300   Co(C.sub.10 H.sub.11 O.sub.5).sub.2                                                     reddish                                                                       brown                                            amyl  Cu.sup.2+                                                                           >172   Cu(C.sub.11 H.sub.13 O.sub.5).sub.2                                                     greenish                                                                      yellow                                           amyl  Co.sup.2+                                                                           >278   Co(C.sub.11 H.sub.13 O.sub.5).sub.2                                                     reddish                                                                       brown                                            isoamyl                                                                             Cu.sup.2+                                                                           >174   Cu(C.sub.11 H.sub.13 O.sub.5).sub.2                                                     greenish                                                                      yellow                                           isoamyl                                                                             Co.sup.2+                                                                           >265   Co(C.sub.11 H.sub.13 O.sub.5).sub.2                                                     reddish                                                                       brown                                            allyl Cu.sup.2+                                                                           >169   Cu(C.sub.9 H.sub.7 O.sub.5).sub.2                                                       greenish                                                                      yellow                                           allyl Co.sup.2+                                                                           >300   Co(C.sub.9 H.sub.7 O.sub.5).sub.2                                                       reddish                                                                       brown                                            cyclohexyl                                                                          Cu.sup.2+                                                                           >227   Cu(C.sub.12 H.sub.13 O.sub.5).sub.2                                                     greenish                                                                      yellow                                           cyclohexyl                                                                          Co.sup.2+                                                                           >260   Co(C.sub.12 H.sub.13 O.sub.5).sub.2                                                     reddish brown                                    cyclohexyl                                                                          Sb.sup.2+                                                                           124-127                                                                              Sb(C.sub.12 H.sub.13 O.sub.5).sub.2                                                     light yellow                                     nonyl UO.sub.2.sup.2+                                                                     169-180                                                                              UO.sub.2 (C.sub.15 H.sub.21 O.sub.5).sub.2                                              orange                                           decyl Cu.sup.2+                                                                           >193   Cu(C.sub.16 H.sub.23 O.sub.5).sub.2                                                     greenish                                                                      yellow                                           __________________________________________________________________________

EXPERIMENT Experiment 1

Run (1) In 100 ml of water was dissolved 0.406 g of cobalt chloride.1.15 g of ethyl comenate was dissolved in 100 ml of chloroform. Eachsolution was combined and shaken sufficiently. In the intersurfacebetween the chloroform layer and the water layer, the formation ofpowdery crystals of the cobalt chelate compound of ethyl comenate wasobserved. The crystals were filtered and dried to yield 0.6 g (46.1%) ofthe cobalt chelate compound of ethyl comenate. The pH of the water layerwas kept at 6.0 using 1% aqueous ammonium solution during the shaking.

The cobaltic ion content in the water layer of the solution from whichthe crystals were removed was measured using an atomic absorptionspectrophotomer. As a result, it was confirmed that 24.7% of cobalticion based on the original cupric ion content remained.

Further, the chloroform layer was condensed and dried to yield 0.38 g(29.2%) of the chelate compound.

Runs (2)-(9) The above run (1) was conducted except for using variousalkyl comenates and metal compounds as shown in the table below.

The results obtained are shown in Table 4 below.

                                      TABLE 4                                     __________________________________________________________________________                   Amount                   Residual                                                                              Weight                                 Amount of                                                                           of            Amount                                                                              Amount                                                                             Metal   of    Yield of                   Alkyl of                                                                            Alkyl Chloro-       of Metal                                                                            of   Content in                                                                            Crystals                                                                            Chelate                 Run                                                                              Alkyl Comenate                                                                            form Metal    Compd.                                                                              Water                                                                              Water Layer                                                                           Formed                                                                              Compd.                  No.                                                                              Comenate                                                                            (ml)  (ml) Compd.   (g)   (ml) (%)     (g)   (g)                     __________________________________________________________________________    1  C.sub.2 H.sub.5                                                                     1.15  100  CoCl.sub.2                                                                             0.406 100  24.7    0.6   0.38                                                                    (46.1%)                                                                             (29.2%)                 2  C.sub.3 H.sub.7                                                                     1.24  "     "       "     "    10.0    Non   1.27                                                                          (90.0%)                 3  C.sub.8 H.sub.17                                                                    1.68  "     "       "     "    1.0      "    1.84                                                                          (99.0%)                 4  C.sub.2 H.sub.5                                                                     1.15  "    Ni(CH.sub.3 COO).sub.2.                                                                0.78  "    21.0    0.98  0.05                                        4H.sub.2 O                  (75.3%)                                                                             (3.7%)                  5  C.sub.3 H.sub.7                                                                     1.24  "      "      "     "    8.0     Non   1.30                                                                          (92.0%)                 6  C.sub.8 H.sub.17                                                                    1.68  "      "      "     "    0.8      "    1.84 (99.2%)            7  C.sub.2 H.sub.5                                                                     1.15  "    CuSo.sub.4.5H.sub.2 O                                                                  0.78  "    25.0     "    1.00                                                                          (75.0%)                 8  C.sub.3 H.sub.7                                                                     1.24  "      "      "     "    4.7      "    1.37                                                                          (95.3%)                 9  C.sub.8 H.sub.17                                                                    1.68  "      "      "     "    0.2      "    1.87                                                                          (99.8%)                 __________________________________________________________________________

Experiment 2

The solubility of various cobalt chelate compounds formed from variousalkyl comenates in various hydrophobic organic solvents saturated withwater at 25° C was measured using an atomic absorption spectrophotomer.

The rsults obtained are shown in Table 5 below.

                                      TABLE 5                                     __________________________________________________________________________            Solvent                                                                                     1,2-di-                                                                            Methyl                                             Chelate               chloro-                                                                            Ethyl                                                                             Octyl                                                                              Chloro-                                   Compound                                                                              Benzene                                                                            Toluene                                                                            Xylene                                                                            ethylene                                                                           Ketone                                                                            Acetate                                                                            form                                      __________________________________________________________________________    Methyl                                                                        Comenate-Co                                                                           -    -    -   -    -   -    -                                         Ethyl                                                                         Comenate-Co                                                                           -    -    -   -    -   -    -                                         Propyl                                                                        Comenate-Co                                                                           ++   ++   ++  ++   +   +    ++                                        Octyl                                                                         Comenate-Co                                                                           ++   ++   ++  ++   +   +    ++                                        __________________________________________________________________________     ++ : Soluble (1 g/1-10ml)                                                     +  : Slightly soluble (1 g/10-30 ml)                                          -  : Very slightly soluble (1 g/100-1000 ml)                             

Experiment 3

1.5475 g of copper solfate was dissolved into 200 ml of water to preparea aqueous solution of a pH of 4.4. 3.34 g of octyl comenate wasdissolved in 200 ml of chloroform. 20 ml portion of each solution wascombined and sufficiently shaken. The pH of the water layer was 2.1. Thewater layer was separated from the chloroform layer, and the cupric ioncontent in the water layer was measured using.

The above experiment was conducted except for adjusting the pH of thewater layer to 3.0, 4.0, 4.5 and 5.0 with a 1% aqueous ammoniumsolution, and the residual content of cupric ion in the water layer wasmeasured.

The results obtained are shown in Table 6 below.

                  TABLE 6                                                         ______________________________________                                                        Cupric Ion Extraction Ratio                                   pH of Water Layer                                                                             From Water Layer (%)                                          ______________________________________                                        2.1             16.7                                                          3.0             53.7                                                          4.0             81.9                                                          4.5             89.4                                                          5.0             99.8                                                          ______________________________________                                    

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A method for recovering metal ions in aqueousliquid which comprises maintaining the pH of the aqueous liquid at notless than 3 and contacting said aqueous liquid containing metal ionswith an effective recovering amount of a comenic ester wherein thealcohol group contains at least 3 carbon atoms in a hydrophobic solvent.2. A method of claim 1, wherein the metal ion is a member selected fromthe group consisting of Fe, Cu, Cd, Pb, Ni, Co, Zn, Au, Pt, Al, Sb, Bi,Sn, Cr, Y and UO₂.
 3. A method of claim 1, wherein the comenic ester isa member selected from the group consisting of a lower alkyl, middlealkyl, higher alkyl, aralkyl and cycloalkyl ester of comenic acid.
 4. Amethod of claim 1, wherein the hydrophobic solvent is a member selectedfrom the group consisting of benzene, toluene, xylene, chloroform,1,2-dichloroethane, and trichloroethylene.
 5. A method of claim 1,wherein the pH is from 4 to
 7. 6. A method for recovering metal ions inaqueous liquid which comprises contacting a comenic ester wherein thealcohol group contains at least 3 carbon atoms and said ester isselected from the group consisting of a lower alkyl, middle alkyl,higher alkyl, aralkyl and cycloalkyl ester of comenic acid dissolved ina hydrophobic solvent selected from the group consisting of benzene,toluene, xylene, chloroform, dichloroethane and trichloroethylene withthe aqueous liquid containing metal ions, with the pH of the aqueouslayer being maintained at not less than three.
 7. A method of claim 6,wherein the comenic ester is a C₅ -C₈ aliphatic hydrocarbon ester andthe solvent is 1,2-dichloroethane and the pH is four to seven.
 8. Amethod of claim 6, wherein the comenic ester is amyl ester and thesolvent is 1,2-dichloroethane and the pH is four to seven.
 9. A methodof claim 7, wherein the comenic ester is octyl ester and the solvent is1,2-dichloroethane and the pH is four.
 10. The method of claim 1,wherein the alcohol group of said comenic ester is propyl, butyl,isobutyl, amyl, isoamyl, octyl, nonyl, lauryl, myristyl, benzyl orcyclohexyl.
 11. The method of claim 6, wherein the alcohol group of saidcomenic ester is propyl, butyl, isobutyl, amyl, isoamyl, octyl, nonyl,lauryl, myristyl, benzyl or cyclohexyl.